The detection principles of TOC (Total Organic Carbon) cleaning verification swabs are mainly based on the following aspects:

I. Principle of Organic Carbon Oxidation

1. Combustion Oxidation Method
   This is a commonly used approach. After collecting samples with TOC cleaning verification swabs, during the detection process, the organic carbon compounds in the samples are completely oxidized under high temperature (usually between 680 – 950 °C) and with the help of catalysts (such as platinum, cerium dioxide, etc.). Taking hydrocarbons as an example, during the combustion process, they react with oxygen to form carbon dioxide (CO₂) and water (H₂O). This is a violent oxidation reaction. For instance, the chemical equation for the combustion of a simple methane (CH₄) molecule is CH₄ + 2O₂ → CO₂ + 2H₂O.
   For complex organic compounds, the oxidation process involves the breaking of multiple chemical bonds and the formation of new bonds. However, the final result is the conversion of organic carbon into carbon dioxide for subsequent quantitative detection.
2. Chemical Oxidation Method
   Some TOC detection methods use chemical oxidizing agents to oxidize the organic carbon in the samples. For example, strong oxidants like persulfates are used to oxidize organic carbon compounds into carbon dioxide under acidic or alkaline conditions. This method is relatively mild and does not require a high – temperature environment. However, the oxidation efficiency may vary depending on the type and structure of the organic compounds. Taking persulfate oxidation as an example, under acidic conditions, persulfate (S₂O₈²⁻) decomposes to generate highly oxidizing sulfate radicals (SO₄⁻·). These radicals can attack the carbon – hydrogen (C – H) and carbon – carbon (C – C) bonds in organic carbon compounds, gradually oxidizing the organic carbon into carbon dioxide.

II. Principle of Carbon Dioxide Detection

1. Non – Dispersive Infrared Detection (NDIR)
   After the organic carbon is oxidized into carbon dioxide, non – dispersive infrared detection is a widely used method for detecting carbon dioxide. Carbon dioxide molecules have a strong absorption peak at a specific infrared wavelength (about 4.26 μm). When the gas containing carbon dioxide passes through the NDIR detection chamber, the light emitted by the infrared light source is absorbed by carbon dioxide, causing a change in light intensity. By detecting the degree of light intensity attenuation, the content of carbon dioxide can be quantitatively determined. This method has high sensitivity and good selectivity, enabling accurate measurement of extremely low concentrations of carbon dioxide.
2. Conductivity Detection Method
   When carbon dioxide dissolves in a specific electrolyte solution (such as sodium hydroxide solution), it causes a change in the solution’s electrical conductivity. For example, carbon dioxide reacts with sodium hydroxide to form sodium carbonate (Na₂CO₃) and water (H₂O). This process changes the ion concentration in the solution, thus altering the electrical conductivity. By measuring the change in electrical conductivity, the content of carbon dioxide can be indirectly calculated. However, this method may be interfered by other ions or compounds, and appropriate sample pretreatment and calibration are required to ensure the accuracy of the detection.

III. Indirect Calculation of Organic Carbon Content

1. Law of Conservation of Carbon Mass
   According to the principle of stoichiometry, during the oxidation reaction, organic carbon is completely converted into carbon dioxide. Based on the law of conservation of carbon mass, the amount of organic carbon in the original sample can be calculated inversely from the detected amount of carbon dioxide. For example, if a certain amount of carbon dioxide is detected, according to the molar mass of carbon in carbon dioxide (12 g/mol) and the number of moles of carbon dioxide, the mass of carbon can be calculated, and then the content of organic carbon in the sample can be obtained. This calculation method is the core of TOC detection. It establishes a direct connection between the detection result of carbon dioxide and the content of organic carbon, thus enabling the quantitative analysis of total organic carbon in the sample.